Does anyone know what the default tire is with the published rear end diffs? Most assume its the smallest tire available. I'm looking at 7.3 F250 crew in this example. 5 tire sizes available. I pulled up oe tire diameter. If its smallest then ratio really changes:

245/75R17 31.5" 3.55 4 30
​​​265/75R17 31.7" 3.53 4.27
275/65R18 32.1" 3.48 4.22
275/70R18 33.2" 3.37 4.08
275/65R20 34.1" 3.28 3.97
Tremor
285/75R18 only "4.30" 34.8" Actual 3.89.

3.55 is sterling, 4.30 is dana m275. I used a online tire size change gear ratio calc for #s.

Is this correct or is default a larger size and the smaller tires are actually steeper?

 

245 75 17 on my 4.30 drw


You might be confused. The rear ratio is with respect to the engine revolutions, not tire diameter. Tire diameter does change the wheel rotations with respect to the engine revolutions though. That's what I think you're asking.


The rear differential gear ratio is the true rear diff ratio.

 

You are confusing "effective" ratio with true ratio.

4.30 gears are 4.30 gears, nothing changes hard cut steel.

Same with all the other gears.

NOW effective gear ratio DOES take tire size into account.

 

Take the tire sizes' revolutions per mile and calculate the ratio difference and apply that to the gear ratio.

Example: if you have a 4.30 rear gear and stock tire size is 628 revolutions/mile and you want to increase to a larger tire that is 523 revolutions/mile

Take 628/523 = 1.20 ratio change. Your 4.30 is now a "3.58" (4.3/1.2). or you can also say you now need a "5.16" (4.3*1.2) rear gear to bring you back to stock gearing.

Since these trucks come with several different tire sizes, you have to chose which one you want to use, or just calculate the ratio of revolutions per mile between the tire sizes being compared. Revolutions per mile is what you want to use since it accounts for the true distance traveled. This number needs to be looked up on the tire specifications as it is not the circumference that you want to use in the calculation because the tire has a contact patch. There are also online calculators.

 

As stated above you are conflating “gear ratio” with “effective (some say final) drive ratio”

You have transmission ratio
you have axle/gear ratio (4.30, 3.55, 3.73, etc)
then you have effective ratio which takes transmission, axle/gear, and then adds in tire size.

use this calculator for some fun;

http://www.grimmjeeper.com/gears.html

 

Ok effective ratio vs true. My question was answered.

You go from a xl with 245/75r245 to a tremor your effective ratio changes. Obviously whats in the pumpkin doesn't.
I guess my point was the effective ratio changes more than most realize pending what tires your turning even with shipping tires.

 

I have a hard time wrapping my brain around this... I went with the 3.31's and the GY 275/70R18 wheels which have a 33.2" diameter and 630 revolutions per mile. I have the PSD and only pull a 6000 lb GVWR camper, so didn't figure I'd really even need the 3.55's... but I did start wondering what my actual (effective) ratio is with the slightly larger tires, and what it would look like if I decided to go with a 35" tire when it comes time to replace them. But I'm guessing, like mentioned above, the effective ratio on mine isn't really 3.31, and it's probably quite a bit lower than that?

 

Simple version get the highest numerical gear ratio you can.

If a 10 speed with 3:55 is good. A 10 speed with 4:30 is better.

 

I got a bit brain boggled also. This website does the math for you…

https://tiresize.com/gear-ratio-calculator/

 

I generally agree. 7.3 w 4.30s coming...... some day maybe. Are they building any this week. 🤪

 

I did not feel the tradeoff in mileage was worth the 4K lbs of extra capacity. I drive the truck a lot, 3.73s work perfectly fine. When we tow, we're close to the 14,400 limit of the trailer so still plenty of overhead on capacity.

 

Yup but I can't get 3.73s on a 250.

 

Thanks for the link. But, I'm still not exactly sure where to start? When choosing an F250 with 6.7, 4x4 and 3.31, the XL trim starts with a 245/75R17 tire, and an XLT (my trim) lists the first tire size as a 275/65R18 tire. I understand that the actual (hard cut steel) gear ratio is a 3.31, but (if this makes sense) which tire size would this most relate to to effectively be 3.31 at the tire? Or maybe I am (still) over thinking this?

For example:
When using 3.31 as the stock gear ratio and I start with the smallest XLT 275/65R18 tire option and go to the 275/70R18's, the effective gear ratio is 3.2
When using 3.31 as the stock gear ratio and I start with the smallest 245/75/R17 tire option on the XL F250, and go to the 275/70R18's the effective gear ratio is 3.14.

But:
When using 3.31 as the stock gear ratio and I start with my 275/70R18's and go to the 285/75/R18's, the effective gear ratio is now 3.15.
When using 3.31 as the stock gear ratio and I start with the 245/75/R17's and to to the 285/75R18's, the effective gear ratio is now 2.9.

So, I would assume that when we pick the 3.31 rear end, to get this effectively at the tire, Ford would base this on the smallest diameter tire that is an option?

I don't guess it doesn't really matter at this point... I already have the truck, these are the tires I have on it, and I can't say I have any issues. When it comes time to replace them I will go with either the same size on it now, or bump up to the 35's (like the DuraTrac's that come on the Tremors), but I have been curious, and then when seeing this thread made me decide to ask about my own situation. Just trying to figure out what my effective gear ratio actually is with my set up. Thanks for any help.

 

Is this any help? You have 3.31’s, it came with 275/70/18’s and you want to install 285/75/18’s. Effective gear ratio would change to 3.15’s

 

https://www.ford-trucks.com/forums/1...l#post18983500

Check out the chart. Shows how the 10 speed increases effective ratios over the older 6 speed transmissions, giving a better final drive ratio. You can use it as an example when comparing tire sizes, what used to be effective for a certain tire size to what is effective to that same tire now. With the 3.73 you can accomplish what you used to be able to get out of 4.30. 4.30 is now crazy low with the 10 speeds so much larger tires can be spun around than before.